1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition and a piezoelectric ceramic device composed of the piezoelectric ceramic composition. In particular, the present invention relates to a piezoelectric ceramic composition that is useful as a material for piezoelectric ceramic devices, such as a piezoelectric ceramic filter, a piezoelectric ceramic resonator and a piezoelectric ceramic oscillator, and to a piezoelectric ceramic device composed of the piezoelectric ceramic composition.
2. Description of the Related Art
Hitherto, a lead zirconate titanate, (Pb(TixZr1-x)O3) or a lead titanate- (PbTiO3) based piezoelectric ceramic composition has been widely used in piezoelectric ceramic devices, such as a piezoelectric ceramic filter, a piezoelectric ceramic resonator and a piezoelectric ceramic oscillator. However, the lead zirconate titanate- or the lead titanate-based piezoelectric ceramic composition contains a large amount of lead, which vaporizes as lead oxide during production of the piezoelectric ceramic device, and thereby results in poor product uniformity. Thus, a piezoelectric ceramic composition that contains little or no lead is desired to overcome this problem. In addition, a lower amount of lead is also desirable in view of environmental pollution.
On the other hand, a piezoelectric ceramic composition based on a layered bismuth compound, such as SrBi2Nb2O9, is free of lead oxide and does not cause such problems.
In addition, SrBi2Nb2O9-based materials, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-328865, exhibit a significantly small change in frequency when temperature changes, and therefore have received attention as piezoelectric materials for resonators recently.
While the piezoelectric ceramic device is typically used at a temperature range, for example, from 60° C. to 200° C., those that can be used at a higher temperature of, for example, about 400° C., are desired for use in a resonator. Since the piezoelectric ceramic resonator cannot be used above its Curie point, where it has no piezoelectric effect, the piezoelectric ceramic resonator must has a Curie point higher than operating temperatures.
According to M. J. Forbess et al. (Applied Physics Letters, Vol. 76, 2943, (2000)), SrBi2Nb2O9 has a Curie point of 418° C. and has a lowered piezoelectric effect when used for a piezoelectric ceramic resonator at a temperature close to 400° C. A preferred Curie point in this case is at least 430° C.